CHICAGO, Illinois (AP) -- Police installed video surveillance cameras around town and saw Chicago's murder rate fall to its lowest level in four decades. Now the cops hope to further cut crime by not only watching, but listening, too.

The city is employing new technology that recognizes the sound of a gunshot within a two-block radius, pinpoints the source, turns a surveillance camera toward the shooter and places a 911 call.

Welcome to crime-fighting in the 21st century.

"Instead of just having eyes, you have the advantage of both eyes and ears," said Bryan Baker, chief executive of Safety Dynamics LLC, the company in suburban Oak Brook that makes the systems.

The technology isn't just gaining favor in Chicago, where 30 of the devices have already been installed in high-crime neighborhoods alongside video surveillance cameras. Baker says dozens more installations will follow.

In Los Angeles County, the sheriff's department plans to deploy 20 units in a pilot test, and officials in Tijuana, Mexico, recently bought 353 units, Baker said. Police in Philadelphia, Pennsylvania, and San Francisco, California, are close to launching test programs of their own, and New Orleans, Louisiana, and Atlanta, Georgia, also have made inquiries.

Military use
Safety Dynamics also works with the U.S. Army and Navy, developing systems that could detect a range of sounds such as glass shattering or diesel trucks slowing in an unexpected location.

Some U.S. troops in Iraq already have a similar system that works differently. Designed quickly in late 2003 and early 2004 by the Defense Advanced Research Projects Agency and BBN Technologies Inc. of Cambridge, Massachusetts, a detector known as "Boomerang" can be mounted on the back of a moving vehicle to locate hostile gunfire.

The Safety Dynamics system deployed in Chicago, known formally as Smart Sensor Enabled Neural Threat Recognition and Identification -- or SENTRI -- uses four microphones to triangulate, or zero in, on the shooter.

By contrast, the Boomerang has sensors mounted atop a pole that detect shock and sound waves from a muzzle blast.

In Chicago, police hope the SENTRI system will add momentum to a technology-fueled crackdown on guns and gang violence.

The city in 2004 reduced its homicide rate to its lowest level since 1965 and police seized 10,000 guns -- successes that were in large part credited to a network of "pods," or remote-controlled cameras that can rotate 360 degrees and feed video directly to squad-car laptops.

The SENTRI systems are an addition to that network.

"They have been extremely successful," said Monique Bond, spokeswoman for the Chicago Office of Emergency Management. "We've been able to see the benefits that cameras and advanced technology bring to the community."

Privacy matters
The American Civil Liberties Union in Illinois is concerned about privacy rights being violated by the city's prevalent camera system. Spokesman Ed Yohnka said officers need to be properly trained in monitoring the cameras and only record activity in public spaces, such as sidewalks and streets.

As long as the cameras and SENTRI system are set up in public spaces, they do not violate the law, said Northwestern University Law School professor Robert W. Bennett.

"You don't have much in the way of privacy issues when you're in a public area," Bennett said.

And local officials say it's hard to argue with the results.

"The crime rates in Chicago are the lowest in 40 years. The price of keeping the community safe far outweighs civil liberty issues," Bond said.

Baker stresses that SENTRI is programmed to recognize only gunshots, not record conversations or "bug" private homes.

"There's no mechanism for other sounds like human voices," he said.

SENTRI is the brainchild of Safety Dynamics and Dr. Theodore Berger, director of the Center for Neural Engineering at the University of Southern California.

Each SENTRI contains a library of acoustical patterns, or "sound signatures," which Berger developed over several years. They're used to differentiate gunshots from other noises, such as traffic and construction, by measuring the unique decibel level of a bullet being fired. That way, a gunshot activates the system but a car backfiring does not.

Adding SENTRI to an existing surveillance camera is not cheap, however. The system costs between $4,000 and $10,000 per unit. In Chicago, money forfeited by criminals is used to pay for both it and the accompanying cameras.

As a result, Police Superintendent Phil Cline told a recent U.S. Conference of Mayors meeting, "the drug dealers are actually paying to surveil themselves.